Vacuum gauge



A 1966 w. H. HAYWARD ETAL 3,

VACUUM GAUGE Filed Sept. 5, 1963 FIG. 3

INVENTORS 34 WESLEY H. HAYWARD 3e 35 SHERMAN L. RUTHERFORD BY MMA TORNEYUnited States Patent 3,267,326 VACUUM GAUGE Wesley H. Hayward, MountainView, and Sherman L.

Rutherford, Palo Alto, Calif., assignors to Varian Associates, PaloAlto, Calif., a corporation of California Filed Sept. 5, 1963, Ser. No.306,756 14 (Iiairns. (Ql. 315-111) This invention relates to vacuumgauges and more particularly to ionization gauges for use in theultra-high vacuum range.

During the mid-forties, the only instrument generally available whichwas capable of measuring pressures below torr was the ion gauge. At thattime, the standard gauges consisted of: a hot wire cathode for emittingelectrons which in the course of their journey collide with themolecules of the gas whose pressure is being measured to form positiveions; a positively charged grid surrounding the cathode for acceleratingand eventually collecting the electrons; and, a metallic shellsurrounding the grid for collecting the positive ions, the numbercollected being an index to molecular density, i.e., pressure. The wholearrangement was enclosed within an envelope connected to the systemwhose pressure was to be determined.

Such gauges never indicated pressures below l0" torr and this wasgenerally attributed to a failure at this pressure of the apparatus thenavailable for pumping to ultrahigh vacuum. It was then suggested thatthe failure was one of measurement rather than one of pumping, and thatthere existed a residual current to the ion collector caused byphotoelectrons ejected from the ion collector by soft X-rays produced byelectrons bombarding the grid. 7

Subsequently, various schemes have been devised for minimizing theerrors in measurement attributable to the X-ray effect. In one, thecathode is placed outside the grid, while the collector, now of smallsurface area, is placed inside the grid. In this way the amount ofX-radiation intercepted by the ion collector can be made arbitrarilysmall by making the ion collector arbitrarily small. In another, agrounded metal tube is placed around the collector between the placewhere it passes into the envelope and the grid in order to shield thecollector throughout this length from X-rays. In still another, asuppressor grid is placed next to and made sufficiently negative withrespect to, the ion collector so that photoelectrons emitted by the ioncollector when struck by X-rays are returned to it. The difl'lculty withthis scheme, however, is that, at the same time the ion collector isbeing struck with X-rays to produce photoelectrons, the suppressor gridis being struck with X-rays thereby producing photoelectrons. Since thecollector is positive with respect to the suppressor grid, thephotoelectrons emitted by the suppressor grid are attracted to the ioncollector, giving rise to a negative X-ray effect.

In accordance with the teachings of the present invention, an ionizationgauge is constructed employing a fourth electrode maintained at apotential below that of the cathode. The soft X-rays generated at thegrid cause photoelectrons to be emitted from this fourth electrode.

Some portion of these photoelectrons are energetically t capable ofreaching the collector, giving rise to a component of residual currentopposite in direction to the normal X-ray current. This produces a netreduction in the X-ray limit of the gauge. With appropriate combinationsof gauge geometry and operating voltages, the photoelectric currentreaching the collector may be adjusted so as to just cancel the normalX-ray current, and thereby completely eliminate any X-ray effect.

ACCOlCllngljQlt is the object of this invention to provide an ionizationgauge which completely eliminates any inaccuracies in pressuremeasurement due to X-ray effects. One feature of the present inventionis the provision of an ionization gauge employing a fourth electrode forproducing photoelectrons when struck by X-rays.

Another feature of the present invention is the pro vision of anionization gauge of the above type wherein the fourth electrode is oflarge surface area as, for example, a conductive sleeve member.

A further feature of the present invention is the provision of anionization gauge of the above type which includes a sealed envelopeadapted to be connected to the system whose pressure is to be measured,and wherein the fourth electrode is a conductive coating on the insidewall of the envelope.

Another feature of the present invention is the provision of anionization gauge of any of the above types including means for varyingthe potential of at least one of the electrodes of the gauge.

These and other objects and features of the present invention andfurther understanding may be had by referring to the followingdescription and claims, taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a top view of a novel gauge of the present invention;

FIG. 2 is a fragmentary cross sectional view of a novel gauge of thepresent invention connected to a system whose pressure is beingmeasured, and including the associated circuitry in schematic; and,

FIG. 3 is a fragmentary cross sectional view of another embodiment ofthe present invention.

Referring now to FIGS. 1 and 2, there is shown an ionization gauge 11constructed in accordance with the teachings of the present invention.The ionization gauge 11 includes a short straight thin wire 12 ofconducting material, as, for example tungsten. This wire is the ioncollector electrode 12, the purpose of which is to collect positiveions. Surrounding the collector electrode 12 is a helically wound grid13, as, for example, platinum clad tungsten supported on grid framemembers 14 forming a cage. This grid 13 is the accelerator or gridelectrode, the purpose of which is to accelerate and eventually collectelectrons. Disposed outside the grid is a coiled filament 15, as, forexample, tungsten. The filament 15 is fabricated on a connector 16having a set screw 17 to allow easy replacement of the filament 15without weld-ing. The filament 15 acts as a source of primary electronswhich collide with gas molecules to form positive ions and moreelectrons.

Each of the above electrodes are connected, as, for example, by weldingto lead-in wires 18, 19, 20. The lead-in wires are insulated from eachother by insulator members 21, as, for example, alumina ceramic,sandwiched between a pair of cup members 22, 23. The upper cup member 22is brazed to the insulator 21 and the respective lead-in wire while thelower cup member 23 is brazed to the insulator member 2'1 and a headerassembly 24. The collector lead-in wire 18 is further shielded by aconductive sleeve member 25, as, for example, stainless steel brazed atits lower end to the header member 24.

The header assembly 24 comprises a cylindrical insulating member 26, as,for example alumina ceramic and a flange member 27, of the type shownand disclosed in US. patent application, Serial No. 144,458, filedOctober 11, 1961, and assigned to the same assignee as the presentinvention.

Surrounding the first three electrodes is a conductive sleeve member 28,as, for example, stainless steel secured at one end by welding to thesystem 29 whose pressure is being determined and secured at its otherend by welding to a mating flange member 30.

In operation, the collector 12 is maintained at some negative potential,for example, --45 volts by means of power supply 31, the grid 13 at somepositive potential, for example +130 volts by means of power supply 3 2and the cathode at or near ground, for example, +6 volts by means ofpower supply 33. The sleeve member 28 or fourth electrode is maintainedat some potential below filament or cathode voltage, typically to -200volts by means of power supply 34.

The bulk of the electrons emitted by the cathode 15 are caused by thegrid 13 to oscillate back and forth in the gauge 11 and in the course oftheir journey collide with the molecules of the gas whose pressure isbeing measured to form positive ions and more electrons. These ions arecollected at the highly negative collector 12, the number collectedbeing an index to molecular density, i.e., pressure.

As is now well known in the art, electrons bombarding the grid 13produce low energy X-rays. Some of this radiation strikes the ioncollector 12 and releases electrons from its surface. So far as thecurrent meter in the external collector circuit is concerned, thedeparture of a negative electron from the collector 12 has the sameelfect as the arrival of a positive ion, thus giving rise to inaccuratereadings in pressure.

However, in the present invention, a certain amount of X-ray radiationalso strikes the fourth electrode or sleeve member 28 and thus releaseselectrons from its surface. In the preferred embodiment shown in FIG. 1the fourth electrode 28 is of large surface area and of such potentialthat a certain proportion of the photoelectrons emitted by the sleeve 28are energetically capable of passing through the grid 13 and reachingthe collector 12, thereby giving rise to a component opposite indirection to the normal Xray current. This produces a net reduction inthe X-ray limit of the gauge 11. With appropriate combination of gaugegeometry and operating voltages, the net X-ray limit can be reduced tozero.

The energy with which the primary electrons emitted by the cathode 15strike the grid is determined by the potential difference between thecathode 15 and grid 13. But the energy of these electrons alsodetermines X-ray energy distribution which in turn determines the energydistribution :of the photoelectrons emitted when the X-rays strike thecollector 12 and the fourth electrode 28.

It has been observed that as the potential difference between cathode 15and grid 13 is increased, the number of photoelectrons energeticallycapable of reaching the collector 12 from the fourth electrode 28 ofFIG. 1 increases less rapidly than the number of photoelectrons .leavingthe collector 12. Therefore, assuming the reverse X-ray current does notquite cancel all the normal X-ray current, than either decreasing thegrid potential as,

- for example, by varying the resistance of variable resistor 36 orincreasing the cathode potential as, for example, by varying theresistance of variable resistor 37 will tend to reduce the X-ray limitof the gauge to zero.

Also, it has been observed that by increasing the potential differencebetween the collector 12 and the fourth electrode 28, either byincreasing the collector potential as, for example, by varying theresistance of variable resistor 38 or decreasing the fourth electrodepotential as, for example, by varying the resistance of variableresistor 39, more photoelectrons emitted from the fourth electrode 28are energetically capable of reaching the collector.

A11 ionization gauge of the type shown in FIG. 1 has been constructedand is capable of pressure measurements to 1.5 1O torr. Also,sensitivity is increased by 50% 4 since because of the negativepotential of the fourth electrode 28, electrons spend a greater timewithin the grid cage producing more ions for a given pressure.

Referring now to FIG. 3, wherein like numerals refer to like parts, theionization gauge 11 includes a sealed glass envelope 40 surrounding thecollector 12, grid 13, and cathode 15, and is adapted to be connected tothe system whose pressure is to be measured. The fourth electrode is aconductive coating 28', as, for example, stannic oxide on the insidewall of the glass envelope.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred for-ms has been only by way of example andthat numerous changes in the detail of construction and the combinationand arrangement or parts may be resorted to without parting from thespirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An ionization gauge for measuring pressure within an enclosed systemincluding: four spaced apart electrodes; the first electrode forcollecting ions; the second electrode for accelerating and collectingprimary elec trons, being maintained at a potential positive withrespect to said first electrode; the third electrode for producingprimary electrons, being maintained at a potential intermediate thepotential of the first and second electrodes; and, the fourth electrodeof large surface area for producing photoelectrons caused by X-rayshitting the fourth electrode, said fourth electrode being maintainedbelow the potential of said third electrode, said second and thirdelectrodes spatially interposed between said first and fourthelectrodes.

2. The gauge according to claim 1, wherein said fourth electrode is aconductive sleeve member.

3. The gauge according to claim 1 including a sealed envelopesurrounding said electrodes and adapted to be connected to the systemwhose pressure is being measured, and wherein said fourth electrode is aconductive coating on the inside wall of said sealed envelop 4. Thegauge according to claim 1, including means for varying the potential ofat least one of said electrodes.

5. The gauge according to claim 4, wherein said electrode whosepotential is varied is the first electrode.

6. The gauge according to claim 4, wherein the elec trode whosepotential is varied is the second electrode.

7. The gauge according to claim 4, wherein the electrode whose potentialis varied is the third electrode.

8. The gauge according to claim 1, wherein the electrode whose potentialis varied is the fourth electrode.

9. An ionization gauge for measuring pressure within an enclosed systemincluding: four spaced apart electrodes; a first electrode forcollecting ions; a second electrode for accelerating and collectingelectrons, being maintained at a potential positive with respect to saidfirst electrode; the third electrode for producing primary electrons,being maintained at a potential intermediate the potential of the firstand second electrodes, the fourth electrode for producing photoelectronscaused by X-rays hitting the fourth electrode and being maintained belowthe potential of said third electrode; said second and third electrodesspatially interposed between said first and fourth electrodes; and meansfor varying the potential of at least one of said electrodes.

10. The gauge according to claim 9, wherein the electrode whosepotential is varied is the first electrode.

11. The gauge according to claim 9, wherein the electrode whosepotential is varied is the second electrode.

12. The gauge according to claim 9, wherein the electrode whosepotential is varied is the third electrode.

13. The gauge according to claim 9, wherein the electrode whosepotential is varied is the fourth electrode.

14. An ionization gauge for measuring pressure within an enclosed systemincluding: a first electrode of 5 6 small surface area for collectingions; a second electrode References Cited by the Examiner surroundingsaid first electrode for accelerating and col- UNITED STATES PATENTS'lectmg electrons, being maintained at a potential pos1- tive withrespect to said first electrode; a third electrode 2,721,972 10/ 1955Rthte1n 317-7 external to said second electrode for producing primary 53,001,128 9/1961 Nottmgham electrons being maintained at a potentialintermediate 3,153,744 10/1964 Tomey 313-4 X the potential of the firstand second electrodes and, a fourth electrode of large surface areasurrounding said DAVID GALVIN 'm Exammer' first three electrodes forproducing photoelectrons caused GEORGE WESTBY, Examiner.

by X-rays hitting the fourth electrode, said fourth elec- 1O trode beingmaintained below the potential of said third SCHNEEBERGER j ig jf kelectrode.

9. AN IONIZATION GAUGE FOR MEASURING PRESSURE WITHIN AN ENCLOSED SYSTEMINCLUDING: FOUR SPACED APART ELECTRODES; A FIRST ELECTRODE FORCOLLECTING IONS; A SECOND ELECTRODE FOR ACCELERATING AND COLLECTINGELECTRONS, BEING MAINTAINED AT A POTENTIAL POSITIVE WITH RESPECT TO SAIDFIRST ELECTRODE; THE THIRD ELECTRODE FOR PRODUCING PRIMARY ELECTRONS,BEING MAINTAINED AT A POTENTIAL INTERMEDIATE THE POTENTIAL OF THE FIRSTAND SECOND ELECTRODES, THE FOURTH ELECTRODE FOR PRODUCING PHOTOELECTRONSCAUSED BY X-RAYS HITTING THE FOURTH ELECTRODE AND BEING MAINTAINED BELOWTHE POTENTIAL OF SAID THIRD ELECTRODE; SAID SECOND AND THIRD ELECTRODESSPATIALLY INTERPOSED BETWEEN SAID FIRST AND FOURTH ELECTRODES; AND MEANSFOR VARYING THE POTENTIAL OF AT LEAST ONE OF SAID ELECTRODES.